This invention relates to multipoint communication systems employing current-loop communications and, more particularly, to a method of and apparatus for detecting and circumventing malfunctions in such systems.
Current-loop communication is utilized principally in point to point communications for computer peripherals and control systems. Normally, an active current source is used to provide an indication of an idle line; whereas, the beginning of transmission of data is indicated by a start bit of zero current. In multiport communications systems in which several devices share a common line, only one device may transmit data signals at a time. If the system employs one long communication line and several short distribution lines, there are two basic approaches to the distribution. One approach is the receive and retransmit daisy-chain technique wherein each unit receives the transmission and passes it to the next unit. The other approach uses a single unit to receive the signal and to distribute it to many short lines connected to the final units. The use of conventional current-loop circuits in either of these approaches creates a communications problem in that a break in one of the lines or failure of the device connected thereto presents a condition in which it appears that the device is actually transmitting data, since no current is provided to the receiver. This failure ties up the entire system because of the one-at-a-time transmitting requirement.
Therefore, it is an object of this invention to provide a reliable and accurate method of and apparatus for use in a multiport current-loop communications system for detecting whether a malfunction has occurred in a respective line or the device associated therewith and for disabling the faulty line to prevent interference with the remainder of the communications system.